1. Field of the Invention
The present invention relates to Low noise optical amplifier (LNOP) and Optical communication system using LNOP, and particularly to Low noise optical amplifier and Optical communication system, which are designed to prevent degradation of the signal-to-noise ratio (SNR) at the receiving station by separating the internally generated amplified spontaneous emission (ASE) from the amplified optical signals.
2. Description of the Related Art
Optical amplifiers are widely used in various fields of optical transmission systems such as CATV distribution network, long-haul transmission between central telephone offices, and undersea inter-terrestrial transmission systems between nations. Also, Semiconductor Optical Amplifiers (SOA), which are superior to the fiber amplifiers in terms of integration and low-price, are key elements in various optical switches and wavelength converters.
In an optical amplifier, the optical signal is amplified without optoelectronic(OE) conversion. Thus, a structure of the optical amplifier is much simpler than that of a conventional regenerator and enables high speed transmission.
Particularly, the development of Erbium Doped Fiber Amplifier (EDFA), which enables direct optical amplification over broad wavelength in the 1550 nm region, lead to great innovation in optical communication technique. In addition, Wavelength Division Multiplexing (WDM) technique using the fact that optical signals with different wavelengths have negligible interaction to each other, enabled enormous expansion in transmission capacity.
FIG. 1 is a simplified diagram showing an EDFA (Erbium Doped Fiber Amplifier).
A conventional Erbium Doped Fiber Amplifier consists of a Pump LD (11), a Wavelength Division Multiplexer (WDM) (13), and a spool of Erbium Doped Fiber (EDF) (14).
The Erbium Doped Fiber (14) is usually made by doping Erbium ions into the core of Single Mode Optical Fiber (SMF) of silica. Erbium ion in silica-host has a meta-stable energy level which is capable of emitting 1.55 xcexcm wavelength photons by stimulated emission. That is to say, electrons in the said meta-stable energy level have a relatively long excited-state lifetime (xcx9c11 msec). When an optical signal in the 1.55 xcexcm band is simultaneously present during the lifetime of excited electrons, the optical signal causes a stimulated emission of photon and therefore the optical signal is amplified.
As explained above, it is necessary that Erbium electrons stay in meta-stable state for a signal to be amplified by stimulated emission. In order to excite Erbium electron into meta-stable state, WDM (13) and Pump LD (11) are used. Pump LD (11) is designed to produce high optical power in wavelength of 980 nm or 1480 nm. The wavelength of 980 nm or 1480 nm is preferred due to its high efficiency in transferring Erbium electrons into the excited state.
WDM (13) is an element for simultaneously delivering pump light emited from the pump LD (11) and optical signal introduced through the input port (12) to the EDF (14).
Irrespective of signal amplification by stimulated emission, Erbium in the excited state always produces spontaneous emission, and this spontaneously emitted photon itself also produces stimulated emission as it propagates along EDF (14). Therefore, in the output port (15) of an amplifier, there always exist amplified spontaneous emission (ASE) along with amplified signals. Due to the existence of amplified spontaneous emission (ASE), the signal-to-noise ratio (SNR) is degraded at the output port of the amplifier.
FIG. 2 is a schematic diagram illustrating the beat noise between amplified optical signals and ASE spectral components and FIG. 3 is the one illustrating the beat noise between ASE spectral components.
Particularly, the optical spectrum vs. wavelength graph in FIG. 2 shows the formation of beat noise (18) as a result of photoelectric mixing between amplified signals (16) and amplified ASE (17).
Likewise, FIG. 3 shows the formation of beat noise (19) as a result of photoelectric mixing between different ASE spectral components (17), irrespective of the existence of optical signals.
The two kinds of beat noises are similar to the well-known beat phenomena in acoustics. Since the photodetector output current is proportional to the intensity of incident optical signals and not the amplitude thereof (Square-law detector), when two light waves having neighboring frequencies f1 and f2 arrive at a photodetector, the photoelectric current has two signals: one having the sum frequencies of input light (f1+f2); the other having differential frequencies (f1xe2x88x92f2). Generally, frequency of a light source used for optical communication is on the order of 2xc3x971014, which is much greater than the typical electrical bandwidth (Be) of a photodetector. For this reason, the photoelectric signals having the same frequency as the optical signal or the sum of two frequencies (f1+f2) are detected only in the average sense at the receiving end. However, since the difference (f1xe2x88x92f2) of two frequencies (f1, f2) could be less than the bandwidth of a photodetector, there exist signals having the beat frequency (|f1xe2x88x92f2|) at the receiving port. Thus, even though the transmitting end intends to send DC signals, there exist oscillating signal components at the receiving end. Furthermore, in case where the light source has a number of frequency components, differential frequency components are produced due to differential combinations of frequencies. When these frequency components exist all together, randomly fluctuating beat noises are generated. If amplified optical signals and ASE exist together as they do at the output port of the amplifier, two types of beat noises are produced at the same time; signal-spontaneous beat noise which occurs between optical signals and ASE having frequency close to that of the optical signals, and spontaneousxe2x80x94spontaneous beat noise which occurs between ASEs.
These beat noises are the dominant noise sources at the receiving end and cause distortion of signals.
Conventionally, a single channel optical amplifier can be evaluated by gain (G), output saturation power and Noise Figure (NF).
Noise Factor (F) is the input signal-to-noise ratio (SNRin) divided by output signal-to-noise ratio (SNRout), while Noise Figure (NF) is given as 10 times the common logarithm value of Noise Factor (i.e. NF=10log10F). Thus, the fact that noise figure of an optical amplifier can not be lower than 3 dB means that all optical amplifiers degrades the input SNR down to at least half of the original value.
In semi-classical viewpoint, noise figure theories of an optical amplifier can be explained as follows.
First of all, in measuring SNR (signal-to-noise ratio) of optical signals entering through the input port of the optical amplifier, it is assumed that an ideal laser source with a certain wavelength, intensity and bandwidth is directly connected to an ideal detector without any functional or systematical loss.
In this ideal set-up, still there exist two types of noises; thermal noise and laser shot noise. Under the assumption that detector load impedance and temperature are constant, thermal noise is given by a constant irrespective of the intensity of input optical signals. For optical outputs exceeding a certain value, thermal noise becomes negligible compared to laser short noise, and thus can be ignored.
Accordingly, SNRin (input signal-to-noise ratio) is given by the following equation.                               SNR          in                =                              P            s                                2            ⁢                          xe2x80x83                        ⁢            hv            ⁢                          xe2x80x83                        ⁢                          B              e                                                          (        1        )            
(hv: photon energy, Ps: optical power of input signals, Be: bandwidth of detector)
By the equation, it could be found that signal-to-noise ratio of an input signal entering through the input port of the optical amplifier (SNRin) is proportional to the optical power (Ps) of the input signal, or more specifically to the input number of photons per unit time (Ps/hv).
In measuring signal-to-noise ratio of output signals (SNRout), it is assumed that optical source is connected to the input port of an optical amplifier by a lossless jumper cord, and the output of an optical amplifier is similarly connected to an ideal detector without any loss.
Generally, the photodetection current of amplified signals contain thermal noise, shot noises of the amplified signal and ASE, signal-spontaneous beat noise, and spontaneousxe2x80x94spontaneous beat noise.
Since the thermal noise is assumed to be negligible in measurement of SNRin, it can also be ignored in determination of SNRout (output SNR). Additionally, compared to the shot noise and the signal-spontaneous beat noise, the spontaneousxe2x80x94spontaneous beat noise can be significantly lowered by placing a narrow optical bandpass filter in front of the detector.
Accordingly, SNRout (output signal-to-noise ratio) is dominated by the shot noise of the amplified signal and the signal-spontaneous beat noise as given by the following equation.                               SNR          out                =                                            (                              R                ⁢                                  xe2x80x83                                ⁢                G                ⁢                                  xe2x80x83                                ⁢                                  P                  s                                            )                        2                                              2              ⁢              q              ⁢                              xe2x80x83                            ⁢              R              ⁢                              xe2x80x83                            ⁢              G              ⁢                              xe2x80x83                            ⁢                              P                s                            ⁢                              B                e                                      +                          4              ⁢                              R                2                            ⁢              G              ⁢                              xe2x80x83                            ⁢                              P                s                            ⁢                              ρ                ASE                            ⁢                              B                e                                                                        (        2        )            
(q: electron charge, R: detector responsivity, G: gain, Ps: optical power of input signals, Be: bandwidth of detector, xcfx81ASE: noise power spectral density in single polarization)
Therefore, the noise factor of an optical amplifier is given by the following equation.                     F        =                              [                                          SNR                in                            /                              SNR                out                                      ]                    =                                                    2                ⁢                                  ρ                  ASE                                                            G                ⁢                                  xe2x80x83                                ⁢                hv                                      +                          1              G                                                          (        3        )            
Also, since xcfx81ASE is given by xcfx81ASE=nsphv(Gxe2x88x921) for a linear amplifier, Equation 3 is simplified as follows.                     F        =                              2            ⁢                          n              sp                        ⁢                          xe2x80x83                        ⁢                                          (                                  G                  -                  1                                )                            G                                +                      1            G                                              (        4        )            
For large value of gain (G greater than 10), the noise factor is approximately given by F=2nsp. Since spontaneous emission factor (nsp) is always greater than 1, the minimum noise factor is obtained when nsp=1 which corresponds to complete population inversion of the gain medium. The corresponding noise figure is then given as NF=10log10F=10log102=3.01 dB.
This is considered as the lowest noise figure that can be achieved. This implies that every time an optical signal is amplified, the signal-to-noise ratio is reduced to the half. Thus, in optical transmission systems where a number of optical amplifiers are used for long-distance communication or power splitting, maximum achievable gain is limited by gain saturation due to accumulated ASE, and signal distortion by signal-spontaneous beat noise limits the total number of optical amplifiers that can be cascaded in series. In case of a semiconductor optical amplifier (SOA), even though it has many advantages over fiber amplifier such as flexible choice of wavelength, high energy efficiency, low production cost, miniaturization and integration, SOA is not in wide spread use in an optical transmission system due to high noise figure.
As explained above, noise figure is considered as one of the most important characteristics of an optical amplifier, and the only characteristics that is limited by fundamental laws of physics. For these reasons, development of techniques related to lowering the optical amplifier noise figure is expected to have an immense influence in both science and industry. Accordingly, there have been many efforts to improve the noise figure of an optical amplifier.
However, conventional optical amplifiers could not eliminate the degradation of signal-to-noise ratio perfectly. The improvement has been only within the limit of conventional theories such as the maximization of population inversion and ASE filtering by narrow band optical filter.
Such conventional optical amplifiers are disclosed in U.S. Pat. Nos. 5,191,390, 4,809,285, 5,636,053 and 5,403,572 and EP laid-open patents 556,973, 470,497, and 647,000.
One of the most common methods is to divide an optical amplifier into two stages and place an optical isolator between the stages so that the optical isolator may prevent lowering population inversion at the input end by the backward propagating ASE. Using a narrow band pass filter in front of the photoreceiver, spontaneousxe2x80x94spontaneous beat noise can be minimized.
Other methods include making a amplifying section into a curved shape to eliminate ASE which has a bigger propagation loss than the signal; making amplifying fiber core into double structure to absorb remnant pump power; using a polarizer to eliminate ASE having polarization perpendicular to that of the optical signal; and lowering temperature of the optical amplifying system to decrease noises.
However, it should be noted that any of these conventional methods is not intended to overcome the theoretical limit of 3 dB. Thus, development of an optical amplifier capable of effectively separating ASE from optical signals is required to significantly improve the SNR at a receiving port.
The present invention originates from the technical requirement as above, and an object of the present invention is to provide an optical amplifier capable of separating ASE from amplified optical signals.
Particularly, the present invention provides a low-noise optical amplifier with a noise figure less than 3 dB by separating ASE, which is inevitably generated in the course of optical amplification, thus eliminating amplified spontaneous emission shot noise and signal-spontaneous beat noise.
The above and other features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention. Also, the objects and merits of the present invention are to be realized with the means and the combination of the means in the claims.
In order to achieve the goals described above, a low noise optical amplifier according to the preferred embodiment of the present invention includes an optical amplifying element to amplify optical signals with a predetermined wavelength (xcex) entering through one or both of ends (F1, F2), and impose a fixed phase difference on ASE (amplified spontaneous emission) with the predetermined wavelength (xcex), which is generated within the said amplifying element and emitted through both ends (F1, F2); a beam split means to interfere amplified optical signals and ASE from one end (F1) of the optical amplifying element with those from the other end (F2) of the optical amplifying element, the beam split means having four optical ports (D1, D2, D3, D4), in which a beam entering through either the first port (D1) or the second port (D2) splits into the third port (D3) and the fourth port (D4), and a beam entering through either the third port (D3) or the fourth port (D4) splits into the first port (D1) and the second port (D2); a first path of length (L1) optically connecting one end (F1) of the optical amplifying element to the first port (D1) of the beam split means; and a second path of length (L2) optically connecting the other end (F2) of the optical amplifying element to the second port (D2) of the beam split means.
At this time, the path length difference between the first and second path (L1 and L2) is designed so that the amplified optical signals have constructive interference at the third port (D3) of the beam split means and destructive interference at the fourth port (D4), whereas the internally generated ASE having the same wavelength as the signal has destructive interference at the third port (D3) of the beam split means and constructive interference at the fourth port (D4).
The optical amplifier of the present invention may further include a beam separation means to separate optical signals entering through the third port (D3) of the beam split means, to be amplified, from amplified optical signals discharged through the third port (D3).
Preferably, the beam separation means has at least three ports (S1, S2, S3), in which a portion or all of optical signals entering through one port (S1), which is to be amplified, is discharged through another port (S2), while a portion or all of optical signals entering through another port (S2), which is already amplified, is discharged through the other port (S3).
Additionally, the optical amplifier of the present invention may further include a non-reflection means connected to the fourth port (D4) of the beam split means to prevent the ASE from being reflected toward the beam split means.
The beam separation means can be provided on the first or second path (L1, L2). In this case, the beam split means is preferably designed to have a beam splitting ratio to make the intensity of ASE, which enters through the first port (D1) and gets discharged through the third port (D3) of the beam split means, equal to the intensity of ASE, which enters through the second port (D2) and gets discharged through the third port (D3) of the beam split means.
The optical amplifier of the present invention may further include a beam attenuation means located on the first or second path (L1, L2) to attenuate optical signals and ASE so that the intensity of ASE, which enters through the first port (D1) and gets discharged through the third port (D3) of the beam split means, is equal to the intensity of ASE, which enters through the second port (D2) and gets discharged through the third port (D3) of the beam split means.
At this time, the optical amplifying element should be selected so that ASEs with a specific wavelength have a fixed phase difference, not changing by time, at both ends (F1, F2) of the optical amplifying element, and more preferably have same phase. However, the ASEs need not surely have the same phase. If the phase difference between ASEs is constant, the object intended by the present invention can be obtained by adjusting the length difference between the optical paths suitable for the phase difference.
As an example of the optical amplifying element having the above-described characteristics, a DFB (Distributed Feedback) optical amplifying element can be adopted. This optical amplifying element has a grating structure in which first medium having an effective refractive index (n1) with thickness (t1) and second medium having an effective refractive index_(n2) with thickness (t2) alternate regularly along the direction of beam propagation, wherein the grating period approximately satisfies the following equation: (n1xc3x97t1)+(n2xc3x97t2)=mxcex/2, where xcex is a designed wavelength, m is a natural number, and wherein at least one of the first and second medium has gain.
This optical amplifying element shows the most ideal characteristics when m is 1 in the equation. Additionally, the optical amplifying element preferably has a reflection symmetry around the center in order to equalize the phase at both ends of the optical amplifying element. Therefore, both ends and the center of the optical amplifying element are preferably made of the first medium.
Particularly, the optical amplifying element has gain grating structure in which an imaginary part of refractive index has a periodical variation, rather than index grating structure in which gain medium is homogeneously distributed and a real part of refractive index has a periodical variation.
Assuming that the optical amplifying element has the gain grating structure and the first medium is gain medium, the thickness (t1) of the first medium should be much less than the designed wavelength (xcex), and preferably given by the following equation: t1xe2x89xa6xcex/(4n1).
It is desirable that the beam split ratio of the beam split means is 50:50 and the optical path length difference(xcex94L=L1xe2x80x94L2) between the first and second paths is preferably given by the following equation: xcex94L=xcex/(4n0), where xcex is a designed wavelength and n0 is an effective refractive index of the optical paths L1 and L2.
According to another embodiment of the present invention, there is provided a low noise optical amplifier for individually amplifying a plurality of optical signals having different wavelengths (xcex1xcx9cxcexn) based on each wavelength (xcex1; here, 1xe2x89xa6i(integer)xe2x89xa6n), which includes an optical amplifying element array (A1xcx9cAn) having a plurality of optical amplifying elements for each wavelength (xcex1; here, 1xe2x89xa6i(integer)xe2x89xa6n) to amplify an optical signal of a wavelength (xcex1) entering through one or both ends (Fi1, Fi2) and impose a fixed phase difference on ASE (amplified spontaneous emission) with a wavelength (xcexi), which is generated within the amplifying element Ai; first multiplexing and demultiplexing means to multiplex amplified optical signals and ASEs, which are discharged through one ends (F11xcx9cFn1) of the amplifying elements and transmitted along division paths (L11xcx9cLn1) for each wavelength, into one common path (Lc1) and divide a plurality of optical signals with different wavelengths transmitted through the common path (Lc1) based on each wavelength and then demultiplex the optical signals into the division paths for each wavelength; second multiplexing and demultiplexing means to multiplex amplified optical signals and ASEs, which are discharged through the other ends (F12xcx9cFn2) of the amplifying elements and transmitted along division paths (L12xcx9cLn2) for each wavelength, into one common path (Lc2) and divide a plurality of optical signals with different wavelengths transmitted through the common path (Lc2) based on each wavelength and then demultiplex the optical signals into the division paths for each wavelength, the first and second multiplexing and demultiplexing means having divided ports (W11xcx9cWn1 and W12xcx9cWn2) for each wavelength to lead in and discharge each beam with different wavelength, and common ports (Wc1, Wc2) to lead-in and discharge a plurality of beams with various wavelengths; a beam split means to interfere a plurality of amplified optical signals and ASEs having different wavelengths multiplexed by the first multiplexing and demultiplexing means with a plurality of amplified optical signals and ASEs having different wavelengths multiplexed by the second multiplexing and demultiplexing means, the beam split means having four optical ports (D1, D2, D3, D4), in which a beam entering through either the first port (D1) or the second port (D2) splits into the third port (D3) and the fourth port (D4), whereas a beam entering through either the third port (D3) or the fourth port (D4) splits into the first port (D1) and the second port (D2); a first path including the first division path (L11xcx9cLn1) for optically connecting one ends (F12xcx9cLn2) of the amplifying elements to each of the divided ports (W11xcx9cWn1) of the first multiplexing and demultiplexing means, and the first common path (Lc1) for optically connecting the common port (Wc1) of the first multiplexing and demultiplexing means to the first port (D1) of the beam split means; and a second path including the second division path (L12xcx9cLn2) for optically connecting the other ends (F12xcx9cFn2) of the amplifying elements to each of the divided ports (W12xcx9cWn2) of the second multiplexing and demultiplexing means, and the second common path (Lc2) for optically connecting the common port (Wc2) of the second multiplexing and demultiplexing means to the second port (D2) of the beam split means.
At this time, a beam separation means can be provided on the first or second common path (Lc1, Lc2) in order to separate optical signals entering through the common port (Wc1, Wc2) of the first or second multiplexing and demultiplexing means, which is to be amplified, from amplified optical signals and ASEs discharged through the common port (Wc1, Wc2) of the first or second multiplexing and demultiplexing means.
Additionally, when adopting an optical amplifying element (A1) imposing a same phase on the ASE with the certain wavelength at both ends (Fi1, Fi2), the optical path length difference (xcex94Li) between the first and second paths can be given by the following equation: xcex94L1=(Li1+Lc1)xe2x88x92(Li2+Lc2)=xcexi/(4n0), where 1xe2x89xa6i(integer)xe2x89xa6n, Li1 and Li2 are the length of division paths, Lc1 and Lc2 are the length of common path, xcex1 is a designed wavelength, n0 is an effective refractive index of the optical path.
According to another aspect of the present invention, there is also provided an optical communication system adopting the above-described optical amplifier.